MiraLax - I took out of date Polyethylene Glycol 3350, NF Powder for oral solution. 09/13 will it?

No. Most expiration dates are placed as a quality measure... the point after which the manufacturer can no longer ensure that at least 90% of the drug is still active. Very few drugs degrade into toxic byproducts. Aspirin for example should not be taken after the expiration date as it turns into vinegar/acids.